Process for making stainless steel



Patented Aug. 29, 19 33a UNITED STATES PATENT OFFICE N Drawing.

Application December 29, 1931 Serial No. 583,789

2 Claims.

My invention relates to the manufacture of chromium-steel alloys,particularly the alloys containing chromium or chromium and nickelcommonly known as stainless steel or rustless 5 iron.

Chromium alloy steel such as is termed stainless steel has under theprevailing practice been made by the use of low carbon ferro-chromiumwhich is relatively costly to produce and is made use of in a processwhich is relatively expensive.

Such alloy usually contains ten to twenty percent of chromium, sometimesfive to twelve percent of nickel, not over .06 to .15 of one percent ofcarbon, the balance mostly iron. As carbon cannot be reducedafterlchromium has been introduced into molten iron without loss ofchro- 1 mium, it has been considered necessary that the carbon contentin the ferro-chromium introduced into the molten iron be very low.

It is the object of my invention to avoid the necessity of using lowcarbon ferro-chromium and to permit the use of chromium ore by reducingthe chromium direct from the ore so as to produce chromium alloy steeland chromium nickel alloy steel at less cost than by the prevailingprocess.

With this object in view my invention consists in the processhereinafter described and claimed.

In carrying out my process, iron or steel is melted in an electricfurnace of usual type to form a bath and is treated with an oxidizingflux of lime and iron ore, or lime and mill scale, for reducing thecarbon in the molten bath to about five to ten one-hundredths of onepercent de pending upon the percentage of carbon desired in thechromium, or chromium-nickel, steel. As soon as the carbon in the moltenbath has been reduced to the desired degree the oxidizing slag is rakedor poured off and a slag of aboutfour parts of lime to one partfluorspar is added in quantity sufficient to cover the surface of themolten bath to a depth of two or three inches when the slag is treatedand the metal of the bath is brought to a good workable temperature.

The molten metal of the bath being thus covered with slag'of thecomposition stated, a mixture of chromite, silicon-chromium alloy, limeand fluorspar is slowly charged on top of the layer of slag, that is,charged not faster than the wchromium content of the mixture is thenbeing reduced by the silicon in the silicon-chro- Inium alloy.

The silicon-chromium alloy preferred contains fifty-six percent ofsilicon, twenty-seven percent of chromium, six one-hnndredths percent ofcarbon and the balance mostly iron. Chromite contains from forty-five tofifty-four percent of chromium oxide CrzOs, and 8 to 15 percent iron.The chromite'and the silicon-chromium alloy are both ground fine enoughto pass an 8 to 60 mesh screen while the lime and fluorspar are left inlumps of about chestnut size. All of the ingredients are intimatelymixed.

The percent of silicon and the percent of chromium alloy may be variedfrom five to ten percent above or below the percentage men-. tioned.

When slowly charged into the slag as above described the silicon willreduce the chromite, picking up the oxygen and leaving the chromium andiron free to pass into the molten metal below the slag, the siliconbeing oxidized to silica remaining in the slag.

After the charging of the mixture has been kept up until the desiredamount of chromium has been reduced and added to the molten iron abouteighty percent of the slag is removed and lime and fiuorspar added toform a refining slag and after a short time the alloy is ready to bepoured into a ladle to be cast into ingots.

In practical operation in an electric furnace of the common arc type, Ihave first melted 1100# of steel scrap and reduced the carbon to 116% bymeans well known to the art. After adding lime and fluorspar to form aslag covering, I have charged a mixture of 500# chromite containing 50%CI2O3, 12% FeO, 150# silicon-chromium alloy containing 56% silicon, 27%chromium and 12% iron, 350# burnt lime, and 60# fiuorspar, the chromiteand silicon-chromium alloy being finely ground and as a result, haveproduced a fine quality of chromium alloy steel of the followinganalyses: chromium 15%, silicon 20%, carbon 09%, manganese 29%, at aless cost than by the use of low carbon ferrochromium as in the generalprocess of making stainless steel. I have also made the so-called 18-8stainless steel, that is 18% chromium and 8% nickel by adding nickel tothe plain low carbon steel bath before the slag covering was 100 added,then following the same procedure as for plain chromium alloy steel. Ineach case the chromium loss has not exceeded 6% of the amount chargedand the carbon held well under .1o% in the steel.

Having thus described my invention what I claim is:

1. A process of making an alloy containing iron and chromium whichconsists in forming a bath of molten iron, reducing its carbon content110 by use of an oxidizing iiux, removing the oxi-' dizing slag from thesurface of the bath, protecting the bath of molten metal by adding limeand fiuorspar to form a coating of slag of substantial thickness on thesurface of the molten metal, and charging on top of the slag so formed amixture of chromite and silicon-chromium al- 10y in pulverized form withlime and fiuorspar, the silicon-chromium alloy containing approximatelytwo parts of silicon to one of chromium.

2. A process of making an alloy containing iron and chromium whichconsists in forming a. bath of molten iron, reducing its carbon contentALBERT F. PLOCK.

